The present invention relates to a semi-transparent monitor detector structure for monitoring the emission of surface emitting light emitting devices, preferably vertical cavity surface emitting lasers (VCSEL). Surface emitting light emitting devices, to include surface emitting lasers, have many advantages over conventional edge emitting devices. These advantages includes less complicated manufacturing processes, single mode as well as multimode operation higher coupling efficiencies and overall lower cost. Vertical cavity surface emitting lasers (VCSEL) are one promising surface emitting device which can be used in a variety of communication systems. Like other lasers used in fiber optic communications, a feedback circuit is desired to assure a near constant output power from the laser be maintained. To this end, during operation, ambient temperature changes and aging of the device can result in fluctuations in the output power of the laser. In order to provide proper feedback to assure a near constant output, some of the light emitted from the laser must be directed to a photo detector. The output of the photo detector is transmitted to a control circuit which adjusts the driver circuitry and thus the current to the laser until the desired output power is regained.
In edge emitting devices, the rear facet of the edge emitting laser diode allows for monitoring of the optical power emitted from the front facet of the laser diode to assure that necessary adjustments to the injected current to the laser be maintained for constant output power. This type of monitoring of an edge emitting device through the rear facet of the laser diode is a very efficient means by which monitoring is accomplished, as this type of monitoring does not interfere with the output of the laser diodes front facet. Again, a feedback control circuit is used to detect any changes in the optical power of the front facet of the laser diode and controls the transmitter driver circuit to adjust the laser diodes drive conditions such that the optical power is kept at a constant desired level. Unfortunately, in vertical cavity surface emitting devices, there is no rear facet only a rear mirror structure which will not normally allow much light to pass.
The advantages of vertical cavity surface emitting lasers has led to some solutions for monitoring the power output from a surface emitting laser. To this end, U.S. Pat. No. 5,491,712 to Lin et al. as well as U.S. Pat. No. 5,475,701 to Hibbs-Brenner, the disclosures of which are specifically incorporated herein by reference, teach various ways to monitor the output of a VCSEL. The reference to Hibbs-Brenner monitors light output from the backside of the device, which is normally the 10-20% of the total light output of the device. This 10-20% portion of the light emitted from the backside is normally not coupled to any waveguides and is therefore lost. The device disclosed in the reference to Hibbs-Brenner is an AlGaAs/GaAs vertical cavity surface emitting laser which has a photodiode made of similar III-V materials fabricated beneath the multiquantum well structure of the laser. This is a rather complicated structure, and requires a rather complicated photodetector, generally a PIN structure. Another drawback to the device of the Hibbs-Brenner structure in addition to being complex to manufacture, and thereby costly, this structure also requires that the photodetector is biased to monitor the detected light output. Additionally, in array form, the integral detector structure has to be formed for each channel which can be a costly processing step, and a potentially yield issue. If one feature of the integrated device fails the whole array is rejected. The uniformity in performance and reliability of the integrated structures of the prior art devices are not clearly understood. The integrated structure does not necessarily guarantee detection and monitoring of the different mode field emissions from the VCSEL and therefore can not give a direct feedback on the power of the beam launched into the fiber. The multimode nature of the VCSEL are essential for data link applications due to "modal noise sensitivity" in data communication links. The integrated detector devices of the references to Hibbs-Brenner and Lin, et al. only detect the field developed inside the cavity. The mode field power inside the cavity is unreliable in part because of the presence of the amount of optical mode field power inside the cavity as a result of spontaneous emission. The spontaneous emission from the cavity is not the desired coupled energy to the fiber for communications, and thus is not an accurate measure of the operating condition of the device. Other methods for monitoring the beam power are reflective and refractive methods by which a detector sees the partial reflection of the VCSEL beam. The multimode nature of the VCSEL does not guarantee the direct monitoring of all the power emitted from the VCSEL. This is because only some of the many different mode field emission pattern from the VCSEL (which vary at different operating conditions) will meet the reflection angle requirement of the reflective optics, and the monitor diode used in this scheme will not be able to sense the combined multimode optical field which is incident on to the fiber or waveguide. In summary, the multimode nature of the VCSEL can result in a variation in mode structure and different power and coupling schemes for optical power coupling into the fiber. What is needed is a structure that gives a better correlation for that purpose.
The patent to Lin et al. sets forth two basic structures for monitoring light output from the vertical cavity device. In one embodiment, a photodiode structure is integrated with the surface emitting laser a current wherein a side absorption photodiode structure is used. This side absorption photodiode structure is integrated with a surface emitting laser, where the side photodiode is formed in the region adjacent to the surface emitting laser. Accordingly, this structure relies on the indirect coupling of light from the laser to the photodiode to assure monitoring. In another embodiment in the '712 reference, a schottky photodiode is located in the optical path of the light emitted from the cavity of the laser. The metal layer used for the schottky barrier is chosen so that the schottky barrier formed between the metal and the semiconductor is less than the photon energy of the light to be emitted. The metal is chosen to be of a thickness so that the light is partially transparent to the emitted light. While less complicated than the photodiode structure discussed above in connection with the '712 reference, this structure also suffers from the problems of the integrated detector and also requires the biasing of the detector. Furthermore, the devices of both of the above incorporated U.S. Patents would be very difficult to produce at large scale manufacturing levels.
What is needed is a less complicated device structure for monitoring the output power of a vertical cavity surface emitting laser, as well as other surface emitting light emitting devices, which insignificantly interferes with the output of the device while providing a reliable monitoring of the light energy actually coupled to the fiber in a scheme which is both easy to manufacture, of low cost, and remaining relatively simple in structure.